A. Field of the Invention
The invention relates generally to the field of laminate structures and particularly to hybrid composite laminates resistant to impact, residual stresses, and environmental degradation.
B. Description of the Prior Art
Structures formed of adhesively bonded fiber/resin and fiber/metal matrix composites have recently come into use as high strength, low weight replacements for structures previously formed of cast, stamped, or forged metals. In the aircraft industry in particular, these laminates have found numerous uses due their higher strength to weight ratios and resistance to corrosion and surface degradation.
These laminates have previously been subject to several problems not encountered in the use of metal structures, the cracking of the laminate along matrix stress lines when struck being perhaps the major fault of these structures. Attempts to overcome this and other problems have usually involved orienting the essentially anisotropic or "unidirectional" fibers in several directions in order to produce essentially isotropic structural character within the structure. However, since fiber composites exhibit greatest strength when the direction of the loading and the longitudinal axes of the fibers are coincident, this practice reduces the efficiency of the composite structure and tends to introduce lamination residual stresses comparable to the transverse and shear strength properties of the purely unidirectional composite. These stresses limit the resistance of the laminated structure to mechanical loading, and particularly to thermal and/or mechanical cyclic loading.
The very nature of the prior art laminates also acts to cause reduced resistance to mechanical fracture aside from the nature of the fiber materials themselves. These fiber-reinforced laminates are formed essentially of fibers set in a rigid matrix material, thereby resulting in a material of insufficient flexibility to absorb or dampen energy directed against point-type locations on the structure. A localized impact on the structure respresents a point-like injection of energy, the rigidity of the matrix material instantaneously transferring the injected energy through the laminate to the opposite side thereof. Thus, the opposite surface is placed under immediate elongation stress. The interface between the fibers and the matrix material is also subject to stress, these combinations of stresses resulting in interior cracking, crazing, and surface degradation. One serious problem thus previously faced in the field can be generally described as an inability of fiber/matrix composites to resist localized loading of both a mechanical and thermal nature without fracture within the material.
The present invention provides solution to this deficiency in prior art laminated structures by adhesively bonding layers of thin metal foils between the plies of fiber/matrix composites to form a hybrid laminate which exhibits the best characteristics of resin matrix, metal matrix and foil materials. The present hybrid composite laminates provide improved high velocity impact resistance, increased fracture toughness, resistance to surface erosion and corrosion, improved transverse and shear properties, improved resistance to stresses arising from local discontinuities, reduced lamination residual stresses, and increased cyclic load life while also being less expensive to fabricate than prior structures of comparable character.